In the field of flat panel manufacturing, a photolithography machine for fabricating thin film transistors (TFTs) is the main equipment for projecting an image of a photomask pattern onto the surface of a substrate by means of an objective lens assembly. With the rapid development of the flat panel display industry, the size of substrates is continuously increasing. In order to increase the manufacturing yield, objective lens assemblies have evolved from the earlier ones with only one field of view (FoV) to the modern multi-FoV ones. For example, a Generation 6 (G6) objective lens assembly has six FoVs, i.e., consisting of six objective lenses. A movable shutter is a mechanism for static exposure by illumination, shading of mask marks, etc., and the conventional movable shutter is disposed under an illumination device. With a Generation 4.5 (G4.5) movable shutter as an example, as shown in FIG. 1, it consists of four shutter blades, i.e., a first Y-directional shutter blade 101, a first X-directional shutter blade 102, a second Y-directional shutter blade 103 and a second X-directional shutter blade 104. During normal exposure, a mask stage and a wafer stage move in the same direction and at the same speed, concurrently with the movable shutter moving in synchronization with the mask stage in order to shade marks on the photomask, in which the first and second X-directional shutter blades 102, 104 control an X-directional size of the FoV, and the first and second Y-directional shutter blades 101, 103 control a Y-directional size of the FoV. During static exposure, a slit defined by the first and second Y-directional shutter blades 101, 103 performs a scan in the Y direction. However, when used in the modern multi-FoV scheme, each of the FoV will need to be equipped with such a movable shutter, making the system bulky and complex in structure. Moreover, the multi-FoV scheme requires expansion both in shutter size and shutter stroke. For example, G6 systems employing six objective lenses requires an increase in shutter size from 40 mm for G4.5 system to 750 mm, as well as an increase in shutter stroke from 44 mm for G4.5 system to 1100 mm. Therefore, the conventional movable shutters cannot meet the requirements of modern systems.
Further, referring to FIGS. 2 and 3, for each FoV, the first and second Y-directional shutter blades 101, 103 are required to move in the Y direction during exposure strictly at speeds calculated based on the movement of the mask stage. During normal exposure, one major task for the movable shutter is to shade alignment marks so as to prevent printing the alignment marks on the wafer. Specifically, as shown in FIG. 2, in order to fulfill the mark shading task, a G4.5 movable shutter must be synchronized with a photomask 404 in the following four phases: upon a front mark 404a on the photomask 404 moving into alignment with an illumination device 401 thereabove (as shown in FIG. 2a), the second Y-directional shutter blade 103 moves in synchronization with the photomask 404 while shading the mark until a desired FoV width is achieved (as shown in FIG. 2b); and upon a rear mark 404b on the photomask 404 moving into alignment with the illumination device 401 thereabove (as shown in FIG. 2c), the first Y-directional shutter blade 101 moves in synchronization with the photomask 404 while shading the mark until it reaches a proximal edge of the FoV (as shown in FIG. 2d). During static exposure, one major task for the movable shutter is to work with the illumination device to control an exposure dose that depends essentially on illuminance, attenuation rate, scanning speed and slit width. During static exposure carried out by the G4.5 movable shutter, as shown in FIG. 3, when the photomask 404 moves into alignment with an area to be scan-exposed, the illumination device 401 is activated and the first and second Y-directional shutter blades 101, 103 that define a slit with a desired width move at the same speed for scanning. At the same time, a sensor 605 disposed under the photomask 404 receives light, that has passed through the slit and the mask stage, and obtains data about the exposure.
Therefore, the conventional movable shutters require blade speed control both in normal and static exposure. Moreover, blade speed changes, during which errors are apt to occur, are necessary upon switching between the normal exposure mode and the static exposure mode.